Use the formula $\displaystyle{s}={r}\theta\usepackage{cmbright}\Huge\$ for arc length. In this circle, $\displaystyle{r}={8}\usepackage{cmbright}\Huge\$ cm, and $\displaystyle\theta=\frac{{3}}{{4}}\usepackage{cmbright}\Huge\$ , so

$\displaystyle{s}={8}\usepackage{cmbright}\Huge\$ .